Ju Hahn Lee

Response function of NE213 scintillator for 0.5–6 MeV neutrons measured by an improved pulse shape discrimination

Abstract Using the pulse shape discrimination method combined with the time of flight technique, we have obtained the response function of a 2″ diameter × 2″ thick NE213 scintillator by measuring directly the proton recoil energy spectrum of 0.5–6 MeV prompt neutrons from a 252 Cf source. Three parameters, time of flight (TOF), pulse shape discrimination (PSD) and recoil energy (RE), were recorded in an event-by-event mode with a TOF gate. We attempted to improve the determination of maximum proton recoil energies equal to incident neutron energies by employing two analysis methods. First, we attempted to separate better neutrons from coexisting gamma rays in the PSD spectrum by projecting neutron channels of the PSD spectrum onto both TOF and RE spectra in a cubic matrix constructed by the three-parameter data. The resulting two-dimensional matrix composed of TOF and RE channels was free from gamma rays and corresponding Compton-recoiled electrons, and then projected with neutron energy bins of 0.05 MeV wide determined by TOF. Finally, to determine the maximum proton recoil energies from each RE spectrum with a realistic function, accounting for the nonlinear response of the NE213 scintillator due to the quenching effect, we performed a least-squares fit to the RE spectrum using the four-parameter function. The response function obtained in the present work agrees well with previous experimental results obtained by Gul et al. (Nucl. Instr. and Meth. A 278 (1989) 470) and a Monte Carlo study by Cecil et al. (Nucl. Instr. and Meth. 161 (1979) 439).

Application of Graphite as a Cathode Material for Electrorefining of Uranium

Abstract The conventional electrorefiners to treat a metallic spent fuel equipped with a steel cathode have a sticking characteristic that hinders their overall processing efficiency. The critical question in order to enhance their throughput is how to decrease the sticking coefficient of the cathode. In order to realize this purpose, the conventional steel cathode was replaced with a graphite one. The graphite cathode exhibited self-scraping behavior in which the electrodeposited uranium dendrite falls from the cathode surface on its own without any kind of mechanical operation such as a scraping and rotation of the electrode. This self-scraping phenomenon of the graphite cathode was interpreted to be due to the formation of a uranium graphite intercalation compound. In this self-scraping mechanism, uranium atoms elongate at the graphite’s outermost layer by an intercalation reaction, so the deposited uranium dendrite falls off spontaneously as the gravitational force exceeds the bonding strength of the layers. Based on our preliminary work, a self-scraping should increase the efficiency of an electrorefiner due to the elimination of a mechanical scraping as well as the electrolytic stripping steps of the cathode.

Assessment of a High-Throughput Electrorefining Concept for a Spent Metallic Nuclear Fuel—I: Computational Fluid Dynamics Analysis

Numerical assessment of a high-throughput electrorefining concept for a spent metallic fuel was carried out by using a commercial computational fluid dynamics code, CFX. An electrorefiner concept equipped with a graphite cathode bundle was designed to recover a high-purity uranium product continuously without a noble metal contamination. The performance of the process for the decontamination of a noble metal in a uranium product was evaluated as a function of the process parameters, such as the rotation speeds of the stirrer and the anode basket. The effects of the void fraction of the anode basket cavity and the morphology of the uranium dendrite on the molten-salt flow and collection behavior were also evaluated with the calculated results.

Multitracing Capability of Double-Scattering Compton Imager With NaI(Tl) Scintillator Absorber

The Compton camera can provide 3-D images of radioactive material distribution based on a single measurement at a fixed position. The Compton camera also can image several different kinds of radioactive materials simultaneously, by means of the “multitracing” capability. In the present study, this multitracing capability was tested for a double-scattering-type Compton camera, or Double-Scattering Compton Imager (DOCI), which utilizes two double-sided silicon strip detectors (DSSDs) and one NaI(Tl) scintillation detector. Our experimental result shows that the 137Cs and 60Co gamma sources can be clearly distinguished in 2-D and 3-D Compton images, and that there is no significant interference between the two gamma sources. The imaging resolutions were determined to be 6.2 and 4.7 mm FWHM for the 137Cs (662 keV) and 60Co (1332 keV) point sources at 4 cm, respectively. The angular resolutions, determined from the angular resolution measure (ARM) distributions, were 7.3° and 6.5° for the source energies of 662 and 1332 keV, respectively. The DOCI remains under development; its imaging resolution will be further improved with the incorporation of more sophisticated detectors and the related electronics, including a faster scintillation detector (LYSO) and higher-spatial-resolution position-sensitive detectors.

Assessment of a High-Throughput Electrorefining Concept for a Spent Metallic Nuclear Fuel - II: Electrohydrodynamic Analysis and Validation

Abstract Assessment of a high-throughput electrorefiner for a spent metallic fuel was carried out by using a commercial computational fluid dynamics code, CFX, and its performance was validated experimentally with a surrogate material. An electrorefiner equipped with a graphite cathode bundle was designed to continuously recover a high-purity uranium product without a noble metal contamination. The performance of the process for a decontamination of a noble metal in a uranium product was evaluated numerically as a function of the process parameters such as the rotation speed of the stirrer and the anode basket, and was validated experimentally. The distributions of the electric field and the electrodeposition behavior were also evaluated numerically, and an optimum electrode configuration was suggested.

CIS: A GUI-Based Software System for Monte Carlo Simulation of Compton Camera

Abstract In the present study, a Compton camera simulator based on the GEANT4 detector simulation tool kit and MATLABTM, and designated the Compton Imaging Simulator (CIS), was developed. The software system encompasses a simulator, an image-reconstruction algorithm, and a data analysis tool. The computational time to obtain a sufficient number of Compton scattered data was dramatically reduced using the source-biasing and exponential transform techniques. Also, a four-dimensional Monte Carlo simulation capability was incorporated. A comparison of the simulation results with the experimental results shows that the CIS accurately simulates the Compton camera.

Feasibility study on hybrid medical imaging device based on Compton imaging and magnetic resonance imaging

In this paper, we propose a combined Compton camera/magnetic resonance imaging (MRI) scanner. For this, the table-top Compton camera currently under development in our laboratory is suitable, considering that it is not very large (i.e., a table-top size) and that it uses semiconductor detectors (for both the scatterer and absorber detectors), which in principle are not very sensitive to a magnetic field. The Compton camera takes three-dimensional images from a fixed position and, therefore, does not require a large ring-type structure, making it possible to fit it into an existing MRI system, without requiring major modifications to the system. In the present study, the potential of combining the table-top Compton camera and an MRI scanner for real simultaneous imaging was demonstrated by fusing a Compton camera image of an instance of multi-tracing, generated by using Geant4 Monte Carlo simulations, with an MR image.

Polarization sensitivity and efficiency for a planar-type segmented germanium detector as a Compton polarimeter

Abstract We have investigated performance of a planar-type germanium detector with 25 segments as a Compton polarimeter to measure the linear polarization of gamma rays. We considered three different configurations in sampling horizontal and vertical Compton-scattering events for linearly polarized gamma rays incident on the detector. The experimental polarization sensitivity Q , coincidence efficiency ϵ coin , and figure of merit F for each configuration have been extracted for gamma rays of interest below 600 keV found in our previous in-beam spectroscopy on 155 Gd . We also obtained theoretical values for these three quantities through a Monte Carlo simulation. Experimental values were in good accord with simulation results for all the three configurations. The third configuration, sampling pairs between the segment of incidence and any off-diagonal segments in coincidence, turned out to have the largest Q as well as ϵ coin value of all the three configurations, thereby resulting in the best figure of merit F .

A novel digital pulse-shape analysis for high-resolution position-sensitive Gamma-ray spectroscopy

A novel digital pulse-shape analysis (DPSA) was performed for high-resolution position-sensitive gamma-ray spectroscopy with coaxial-type high-purity germanium (HpGe) detectors. The DPSA consisted of two stages that determined the pulse height, the interaction point, and the time by using the chi-square fitting the digitally recorded pulse shapes with calculated shapes. As the DPSA was applied to gamma-ray spectroscopy using HpGe coaxial detectors, we obtained the energy spectra, the time-difference spectra, and the interaction depth profiles for the gamma rays emitted from 133Ba and 22Na standard sources. The efficacy of the spectroscopic analysis using the DPSA was compared with that using a conventional analog signal process (CASP). Furthermore, we determined the half-lifetime of the 1st excited state in 133Cs by using the DPSA.

Pulse Shape Analysis of a Segmented Planar Germanium Detector by Using the Green's Function Method and Rise-Time Correlations for Depth Information

We performed pulse shape analysis to extract depth information for an interaction point of y rays in a 25-fold segmented germanium detector. With a 90° Compton spectrometer with collimators on both incidence and exit sides, we measured pulse shapes for the two interaction points, (x,y,z) = (2.5 cm, 2.5 cm, 0.5 cm) and (2.5 cm, 2.5 cm, 1.0cm). Experimental pulse shapes were compared with calculated ones by the three-dimensional Greens function method. Analytical solutions for induced charges on each cathode and an anode were obtained for a hole and an electron as a function of time. It is suggested that a rise-time correlation between the anode and the central cathode signals, T 90 (anode) - T 50 (cathode), can be used as a useful means to determine the depth (z) of an interaction point given an in-plane position (x, y). It is shown that calculated rise-time correlations are in qualitative agreement with experimental ones obtained by uncollimated y rays.